Small-angle X-ray scattering functions were measured for six pullulan samples with molecular weights ranging from 2.3 × 104 to 7.4 × 105 in 0.05 M aqueous NaCl at 25 °C and fitted by the perturbed wormlike chain model, comprising touched-bead sub-bodies, to obtain wormlike chain parameters. The parameter values determined were consistent with those determined from previously reported dilute solution properties of aqueous pullulan. Because radii of gyration of not only pullulan polymers, but also pullulan oligomers were consistently explained by the touched-bead wormlike chain model perturbed by the excluded volume effect, the pullulan chain takes a local conformation considerably different from the amylose chain, although both polysaccharides are flexible polymers with an approximately same characteristic ratio.
Narrow distribution pullulan (PUL) samples with different degrees of polymerization (=142−4540) were reacted with octenyl succinic anhydride (OSA) to obtain a hydrophobically modified pullulan (PUL-OSA). The micellar structure formed by PUL-OSA in 0.05 M aqueous NaCl was investigated by small-angle X-ray scattering (SAXS) and fluorescence from pyrene solubilized in the aqueous PUL-OSA solution. SAXS profiles for PUL-OSA solutions were fitted using the flower necklace model consisting of n c unit flower micelles, and n c values estimated by SAXS were almost consistent with results from fluorescence decay of the solubilized pyrene. The micellar structure of PUL-OSA characterized by SAXS and fluorescence was considerably different from that of the alternating copolymer of sodium maleate and dodecyl vinyl ether, P(MAL/C12), an amphiphilic vinyl polyelectrolyte. The flower necklace of PUL-OSA has smaller unit flower micelles and a larger global dimension than the flower necklace of P(MAL/ C12), probably due to differences in the hydrophobic alkyl chain length and the bulkiness of the backbone chain.
This article reviews previous studies on the micellar structure formed by hydrophobically modified polysaccharides in aqueous solutions, by static and dynamic light scattering, small angle X-ray and neutron scattering, and fluorescence from pyrene solubilized in the polymer solution. Those experimental results are consistently explained by the full or loose flower necklace model for pullulan bearing octenyl groups and amylose bearing dodecyl groups, and by the randomly branched polymer model, which is often called as "nanogel," for pullulan bearing cholesteryl groups. We discuss the micellar structures of those hydrophobically modified polysaccharides as well as of an amphiphilic alternating vinyl polymer bearing dodecyl groups, depending on the degree of substitution as well as chemical structures of the hydrophobic moiety and backbone chain.
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